TC locked vs. unlocked... Which for increased mpg?

[turboberserker]

I can't ever remeber this...

I *think* locked = better mileage for some reason.

[joecar]

TCC locked (applied) = better MPG (no slip) and cooler ATF.
TCC unlocked (released) = torque increase and hotter ATF.

(torque converter multiplies torque when slipping, but also loses some power in form of heat).